1. Field of the Invention
The general field of this invention relates to a device for use in introducing ions and/or sample gases to ion detection and/or mobility measurement means in order to allow for greater detection sensitivities in the ion detection and/or mobility measurement means. The invention is used specifically as a coaxial laminar flow, flow tube to be used as an ion source introduction and concentration means in which specific ion species are reacted with a gaseous sample forming ionized species of the gaseous sample, which are then introduced into an ion mobility measurement and detection means. The ion source and introduction method results in a substantial increase in the sensitivity of the ion mobility and detection means. This invention also can be configured for use as a high pressure ion drift tube subsequent to an ion source flow reactor making possible the exclusive introduction of ionized sample molecules into a clean dry drift gas flowing through the drift tube.
2. Prior Art
The use of drift tubes and plasma chromatography for generating and detecting trace gases are known in the art. Typically, particular species of reactant ions are created within the drift tube and the various gate regions within the drift tube help to sort and detect the reactant ion by localizing the reactant ion by drift potential. Cohen, U.S. Pat. No. 3,621,239, discloses a plasma chromatography device in which positive or negative ions are formed by reactions between molecules of a trace gas substance and with primary ions. The positive or negative ions formed, commonly called secondary ions, are separated and detected within the drift cell by utilizing the difference in drift velocity or drift time of the ions of different masses.
Various configurations and alterations of the Cohen device are disclosed and claimed in U.S. Pat. No. 3,626,178, Carroll et al., U.S. Pat. No. 3,626,180 and Carroll U.S. Pat. No. 3,668,383. These four patents disclose improvements on plasma chromatography systems involving the formation of primary or reactant ions and reaction of the primary ions with molecules of trace substances to form secondary or product ions, which may be concentrated, separated, detected and measured by virtue of the difference of velocity or mobility of the ions in an electric field. In general, the inventions described in the above four patents comprise ionizing a host gas, such as air, to produce primary ions. These primary ions then are subjected to an electric drift field causing them to migrate to a reaction space. A sample or trace gas is introduced to the reaction space and the resulting collisions between the primary ions and the trace gas molecules produce secondary ions. The secondary ions, also subjected to the electric drift field are sorted in accordance with their drift velocity or mobility within a plasma chromatograph drift cell.
One disadvantage of the prior art apparatuses and methods for separating, detecting, and measuring trace gases is that the ionization of the host gas and the ionization of the trace gas occur within the drift cell itself. Unwanted bulk unionized host gas, unwanted unionized sample gas, and unwanted ions and reactant species therefore are present within the drift cell, thus preventing accurate measurements of the desired trace gas at levels in the sub-parts-per-trillion range. The unwanted bulk gas species and certain of the unwanted reactants and ion species flow through the drift cell into the measurement and detection devices. The presence of these unwanted components in the drift region results in chemical reactions in this region which prevent the obtaining of sharply focused ion arrival time peaks. Additionally, the structural configuration of prior art apparatuses may allow for host gas ions to travel through the drift cell, thus introducing impurities to the measurement and detection device. The structure of the prior art apparatuses also may allow the host gas ion/trace gas ionization reactions to occur within the drift cell itself, resulting in a dramatic broadening of the arrival time spectra.
The use of plasma chromatography, or ion mobility spectrometry, also is known in the prior art. Hill, Jr. H.H., et al.. Ion Mobility Spectrometry, 62 Analytical Chemistry 23, pp. 1201a-1209a (Dec. 1, 1990), discusses the basic principle of ion mobility spectrometry as carried out using an ion drift tube. The drift tube disclosed in the Hill, Jr., et al. article discloses a counterflow drift tube in which a sample gas introduced to the drift tube is ionized within the drift tube. The ions created then are directed in the typical fashion to the measurement and detection device. As with the patents discussed above, the sample gases are ionized along the axis of the drift region in a non-preferential manner within the drift tube itself, thus introducing the possibility of a wide range of sample gas product ion species and extraneous ions created from other components of the sample gas or its fragments, thus preventing the detection and measurement sensitivity in the sub-parts-per-trillion range.
Atmospheric pressure chemical ionization mass spectrometry APCl/MS and plasma chromatography (PC) have to be extremely sensitive methods for detecting gas phase species at ultratrace levels. Currently, most of these methods are employed only for analysis of bulk phase samples. Therefore, their extreme sensitivity is rather limited to only those species having relatively high proton or electron affinities. The typical hierarchy of potentially stable product ion species present in gas samples limits the present applicability of chemical ionization detection methods to relatively few, very stable species. Sensitive detection of species having relatively low proton or electron affinities can be achieved by coupling atmospheric pressure chemical ionization with a technique, such as gas chromatography (GC), which separates the component(s) of interest in the sample matrix from interfering high affinity species. Thus, a potentially large number of species may be detected with extreme sensitivity using combined GC/APCI/MS or GC/APCI/PC. However, to this date, the powerful capabilities of these combinations of techniques have hardly been exploited. The present invention overcomes these disadvantages by offering a new ion source and sample introduction method for an ion drift tube and a new method of introducing ions of interest into a selectable pure, dry drift gas.